1. Field of the Invention
The present invention relates to an artificial multilayer having a magnetoresistance effect and a method of manufacturing the same.
2. Description of the Related Art
Am electrical resistivity .rho. of a substance, which has a specific value at a predetermined temperature, varies with application of an external magnetic field. This phenomenon is called "a magnetoresistance effect", which is one of galvanomagnetic effects in the same manner as a Hall effect.
This magnetoresistance effect is applied to magnetoresistive elements such as a magnetoresistive field sensor, or a magnetoresistive head (MR head). As a material exhibiting the magnetoresistance effect, a semiconductor and a ferromagnetic material are known.
Since the physical properties of the semiconductor generally vary largely depending on temperature, the upper limit of its operating temperature is restricted to about 100.degree. C. On the contrary, the ferromagnetic material has a small temperature coefficient, and the upper limit of its operating temperature is a Curie point in principle, so that the ferromagnetic material can be used up to much higher temperature as compared with the semiconductor. Further, since the ferromagnetic material can easily be formed in a thin film and miniaturized, a magnetoresistive element made of the ferromagnetic material can effectively detect a magnetic field even if a distance between magnetic charges is as short as .mu.m order.
The magnetoresistance effect of the ferromagnetic material observed when an external magnetic field is relatively weak has a feature that its resistivity varies according to an angle formed between a magnetizing direction and a current direction. This phenomenon is particularly called an anisotropic magnetoresistance effect. The resistivity of general ferromagnetic metal takes maximum when its magnetizing direction is parallel to a current direction (.rho.) and minimum when both are crossed perpendicularly to each other (.rho..perp.). As a quantity of representing the magnitude of the anisotropic magnetoresistance effect, a ratio .DELTA..rho./.rho..sub.0 is used, where .DELTA..rho.=.rho.-.rho..perp., and .rho..sub.0 is the resistivity when an applied magnetic field is zero. As materials having large .DELTA..pi./.pi..sub.0 at a room temperature, Ni-Co or Ni-Fe based alloys are known. Noted that their .DELTA..rho./.rho..sub.0 are no more than about 2.5 to 6.5%.
It has been recently reported that a large magnetoresistance effect is observed in an artificial multilayer in which ferromagnetic layers and nonmagnetic layers are alternatively laminated and magnetization of adjacent ferromagnetic layers are arranged in antiparallel (Phys. Rev. Lett. Vol. 61. p. 2472 (1988)). For example, a multilayer consisting of Fe (a ferromagnetic layer)/Cr (a nonmagnetic layer) system is known. The Fe/Cr multilayer formed on a glass substrate, the maximum relative change of resistivity (.rho..sub.s -.rho..sub.0)/.rho..sub.0, where .rho..sub.0 is the resistivity when an applied magnetic field is zero and .rho..sub.s is the resistivity when the magnetization is saturated, has very large values of -8.4% at a room temperature and -26.4% at 77K (J. App. Mag. Soc. vol. 14, p. 351 (1990)). In such a type of artificial multilayer, however, a saturated magnetic field, i.e. an external magnetic field required to saturate the relative change of resistivity, is 10 kOe or more at a room temperature which much exceeds a practical range required for a magnetoresistive field sensor or an MR head.
Further, it is reported that artificial multilayers other than Fe/Cr system, for example Ni-Fe/Cu/Co/Cu system (J. Phys. Soc. Jap. 59 (1990) 3016) or Ni-Fe/Cu/Ni-Fe/FeMn system (35th Annual Conference on Magnetism and Magnetic Materials, 1990), also exhibit a large magnetoresistance effect.
In these artificial multilayers, an antiparallel aligned state of magnetization which leads to the large magnetoresistance effect, is realized on the way of magnetizing process due to a difference of anisotropies of two types of ferromagnetic layers, that is, a hard layer (a layer having a large magnetic anisotropy) such as Co or FeMn/Ni-Fe, and a soft layer (a layer having a small magnetic anisotropy) such as Ni-Fe (permalloy). The Ni-Fe/Cu/Co/Cu system, however, exhibits a large hysteresis in the magnetoresistance effect with respect to the magnetic field. Therefore, it is required to reduce the hysteresis as small as possible. On the other hand, the Ni-Fe/Cu/Ni-Fe/FeMn system exhibits a small hysteresis in a weak magnetic field up to 15 Oe. Further, its relative change of resistivity .DELTA..rho./.rho..sub.0 varies stepwise to the change of external magnetic field .DELTA.H, that is preferable in practical use. In view of various applications of the magnetoresistance effect, however, it is more preferable to be able to control the relative change of resistivity so as to vary at an arbitrary gradient to the change of external magnetic field, rather than stepwise variation.